1. Field of the Invention
The present invention relates to the use of OFDM (orthogonal frequency division multiplex) in radio telephony systems, and more particularly to a random access protocol applicable to when a mobile station (MS) in a radio telephony system initiates a call, or when a call is handed over from one base station (BS) to another (handover).
2. Discussion of the Background
A random access (RA) protocol defines a method by which a MS initiates contact with a BS, when the BS is not synchronized with the MS. This situation can arise when the MS initiates a call (i.e., when a call is activated) or when a call already in progress is transferred from one BS to another BS.
In an OFDM system, data (information) is modulated onto a broadband signal comprising a large number of individual (discrete) frequency carriers (subcarriers), the aggregate of which form a frequency division multiplex scheme. Bandwidths of the individual sub-carriers, are small and arranged so that the maximum power spectrum of a first sub-carriers represented by sinc(x) (i.e., sinc(x)=sin(x)/x), corresponds with the first minimum in a second sinc(x) function which corresponds to a power spectrum of an adjacent sub-carrier. In each case "x" corresponds to a symbol length of a data symbol modulating the respective carrier.
In other words, the carrier separation equals 1/(symbol length), for rectangular symbols. It is for this reason that adjacent carriers are described as "orthogonal". OFDM systems normally use a FFT (fast Fourier transform) process to demodulate the data signal from the transmitted (carrier) signal. Convolutional forward error coding and FFT techniques may be employed at the modulator (transmitter) stage in order to improve system performance. In the receiver, complementary FFT processing is combined with Viterbi decoding, at the demodulator stage in order to properly decode the information processes by the corresponding techniques at the modulation stage. This ensures that the overall bit error rate is very low. This particular variant of OFDM is known as CD OFDM (Code Division Orthogonal Frequency Division Multiplex). For convenience, in this specification the term OFDM is used to refer to both FD OFDM (frequency divided OFDM) and CD OFDM, unless specific reference is made to either FD OFDM, or CD OFDM.
Using OFDM in the uplink of a multi-access system requires that all mobile stations in the system meet the requirements for orthogonality in the time and frequency domain between sub-carriers. The use of FD OFDM thus requires a random access technique that does not disturb the orthogonality between sub-carriers. Because time synchronization between a BS and an MS is essential to establish and maintain connectivity, it is vital that the delay difference between a MS and a BS be estimated during a random access protocol so that orthogonality between sub-carriers be preserved.
In a random access protocol, a mobile station transmits a known signal sequence to a base station. The base station can lock onto the known sequence, detect it and estimate the time delay.
Multi-access (i.e., multiple users share common radio frequency resources) radio telephony systems are of course well known, e.g. the GSM (Global System for Mobile Communications European Cellular Standard) system, in which a plurality of mobile stations are served via a plurality of base stations connected to a communications infrastructure for controlling individual communications and routing such communications via a land based network, e.g. a PSTN (Public Standard Telephone Network). All such systems require a random access protocol, or a procedure to permit mobile users to enter the network. The use of conventional OFDM for such systems is also known. However, as recognized by the inventors, the requirements of OFDM with regard to sub-carrier orthogonality imposes special problems.
PCT patent application WO 95/07581 describes a method of synchronizing an OFDM QAM (quadrature amplitude modulation), or QPSK (quadrature Phase Shift Keying), receiver when it is first switched on. The transmitted signal power is set at zero for part of the synchronization symbol. During the remaining part of the symbol, the symbol is modulated with a sequence that has optimum auto-correlation properties. The invention is alleged to require only a single symbol for synchronization.
U.S. Pat. No. 5,228,025 discloses a method of synchronization for use in an OFDM system. Certain sub-carriers are omitted, or reduced in power, in the sub-carrier raster (selection). The method is applied to the broadcasting of digital data in multiple channels, notably radio (program) broadcasting, and the synchronization pattern to establish communications is repeated.
PCT patent application WO 92/16063 discloses an OFDM system for broadcasting and receiving digital data within time division multiplexed channels. Each OFDM frame includes frequency reference symbols for synchronization of a receiver with a transmitter.
Synchronization techniques used in OFDM and TDMA systems are also disclosed in U.S. Pat. No. 5,191,576; PCT patent application WO 93/11616; European patent application 0,549,445 A1; and European patent application 0,608,024 A1.
However, none of the cited prior art documents discloses the use, in a digital radio telephony system, of a random access protocol in which a random access sequence can be cyclically repeated, to facilitate connection of a mobile station to a base station upon call initiation or upon call handover